Niten Ichi-ryū(二天一流?), which can be loosely translated as "the school of the strategy of two heavens as one", is a koryū (ancient school), transmitting a style of classical Japaneseswordsmanship conceived by the warrior Miyamoto Musashi. Hyōhō Niten Ichi-ryū is mainly known for the two-sword—katana and wakizashi—kenjutsu techniques Musashi called Niten Ichi (二天一, "two heavens as one") or Nitō Ichi (二刀一, "two swords as one").

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Around 1640, Musashi intended to pass on his art to three successors from among his thousand students; specifically, to Terao Magonojo, his younger brother Kyumanosuke and to Furuhashi Sozaemon. He considered Magonojo to excel in technique but to lack in reflection, while Furuhashi excelled at reflection but lacked technique. Magonojo received the treatise, the Go Rin no Sho. Hosokawa Mitsuhisa made two copies- one for Furuhashi and one for himself, which he transmitted under the name of Ihon go rin no sho. The best known edition today is this Hosokawa copy.

Magonojo then yielded the role of successor to his younger brother Kyumanosuke who had received the Hyoho San-jugo from Musashi. It was Kyumanosuke who transmitted this document to his students with seven added instructions called the Hyoho shiji ni kajo.

Shortly before his death, Musashi also wrote the Dokkodo ("Going My Way"). It seems to be a list of rules that one should try to follow in life steeped in Buddhist precepts.

Terao Kyumanosuke (Motomenosuke) had received the complete transmission of the School of Musashi, with certification and Musashi's two swords. He at first refused to teach and sent what he had received to Musashi's adopted son, Iori. Iori refused the succession, since the honor had not been bestowed upon him. With this, Kyumanosuke then agreed to take over as head—both his and Iori's actions were manifestations of their respect for Musashi.

Succession in the Hyoho Niten Ichi-ryū (the name given by Musashi towards the end of his life) does not follow a hereditary pattern. It is attested to by the bestowing of two artifacts: a scroll on which is written the name of the techniques and the approach to them that must be transmitted if the school is to be perpetuated truly,[3] and a wooden sword that Musashi made himself, with which he trained and used as a walking stick during the last years of his life,[3] today in possession of the city of Usa's Shinto Shrine.

The Gosho-ha Hyōhō Niten Ichi-ryū disputed the lineage claiming that Iwami Toshio Gensho is the sole legal representative of Hyōhō Niten Ichi-ryū until 2007. Miyagawa Yasutaka established a line of Niten Ichi-ryū that continues to practice and thrive in the Kansai region of Japan. Miyagawa Yasutaka and Kiyonaga Tadanao were both students of Aoki Kikuo during the same period. This "Kansai" line, currently under 10th Headmaster Miyagawa Morito, is an alternate but equal lineage to the main line.

According to Gosho Motoharu Hanshi, the Aikuchi roppo was constituted by free nito forms using fukuro shinai, not formalized kata.[4] However this waza was not taught by Musashi and is a later addition.

The Jitte techniques, also according to Gosho Hanshi, are modern addition, and were not practiced in the time of the eighth headmaster, Aoki Kikuo. The techniques taught had a direct relation and followed the order of the grades in the ryu:[4]

Shoden: Itto SeihoChuden: Kodachi SeihoOkuden: Nito SeihoMenkyo: BojutsuMenkyo Kaiden: All the curriculum of the ryu plus have a deep knowledge of the founder teachings.